The invention relates to the coding of a sequence of data bytes, in which two bits of a data byte form a double bit. Each double bit is represented by a time slot frame that has at least four time slots which can assume an on or off value. The invention also relates to a mobile data memory and a reader/writer for carrying out the method, as well as to an identification system having the reader/writer and having at least one mobile data memory.
Non-contacting identification systems operate on the basis of non-contacting transmission techniques. For example, these non-contacting transmission techniques may be based on electromagnetic principles, such as infrared or ultrasound transmissions. Such systems are used, for example, for identifying personnel or moving goods, such as transportation means. Therein, the necessary data is transmitted from a reader/writer to a mobile data memory and back via a non-contacting data transmission path, for example, via an air interface. The non-contacting identification system also allows for acquiring data when the mobile data memory moves by, for example.
In order to use the mobile data memories for an unlimited time, no energy stores, such as batteries, are integrated in the mobile data memories. The electrical power is obtained externally in a non-contacting manner, that is from an electrical or magnetic field that originates from the reader/writer. Therein, as the distance between the reader/writer and the mobile data memory decreases, an increasing amount of energy is drawn from the external field.
For a reader/writer to communicate with such mobile data memories, suitable transmission and coding methods are necessary, which ensure not only that the electronics in the mobile data memory are supplied with power, but also that radio requirements are complied with. Furthermore, in general, only specific frequency bands are allowed for transmitting data. For example, the ISM frequency bands (Industrial, Scientific and Medical) can be used for industrial, scientific and medical applications.
In accordance with, for example, ISO/IEC Standard 15693 Part 2 xe2x80x9cAir Interface and Initializationxe2x80x9d or ISO/IEC Standard 14443, these methods are known as time slot methods for operation in an ISM frequency band.
Methods of this type allow power to be continuously supplied to the data memory electronics. Therein, for the purpose of transmitting power, the carrier frequency, which is modulated with the data to be transmitted, is switched off only for a maximum time interval. Within this time interval, an energy store, which has previously been charged in the mobile data memory, needs to be able to bridge the power supply. Conversely, the data is transmitted from the mobile data memory to the reader/writer by means of loading modulation. In the process, the mobile data memory damps the inducing magnetic field in short intervals. In this loading phase, the mobile data memory requires the maximum power from the previously charged energy store. This loading phase must therefore be as short as possible, since the loading phase has a direct effect on the maximum possible distance between the reader/writer and the mobile data memory for rapid recharging purposes. In accordance with the above Standard, the loading modulation can be continuously carried out for one time slot as the maximum time interval. Alternatively, time slots can also be carrier-frequency-modulated with an auxiliary carrier, so that power can be transmitted within a modulated time slot. Thus, such modulated time slots can follow one another directly (in this context, see also FIG. 2).
In addition, the Standard defines, for a specific carrier frequency, the field strength limit values for the side bands caused by the modulation. The magnitude of the side band modulation is governed predominately by the time ratio and the number of changes from modulated time slots to unmodulated time slots. Therein, the restriction of the maximum permissible field strength in the respective frequency bands leads to a restriction of the maximum data rate for a previously defined, reliable operating distance. At the same time, this means that the maximum possible data rate decreases as the transmission distance between the reader/writer and the mobile data memory increases.
If, for example, in accordance with the above ISO/IEC Standard 14443 Type A, carrier frequency modulation with an auxiliary carrier and Manchester coding for a bit sequence of a data byte, which is to be transmitted, is chosen, then between 8 and 16 changes can occur between modulated and unmodulated time slots, depending on the bit sequence. This will be described in more detail with reference to FIG. 2 below.
This has the disadvantage that the large number of changes between modulated and unmodulated time slots leads to a high level of side band modulation. The resulting maximum possible data rate and/or the maximum possible transmission distance between the reader/writer and the mobile data memory are/is no longer sufficient for many applications.
It is one object of the present invention to provide a coding method that allows for a higher data rate for data transmission and/or a greater transmission distance between a reader/writer and a mobile data memory. Therein, power supply to the mobile data memory and compliance with the side band modulation limits are still ensured.
According to one formulation of the present invention, this and other objects are achieved by a method for coding a sequence of data bytes, in which two bits of a data byte form a double bit. Therein, each double bit is represented by a time slot frame that has at least four time slots. The time slots of the time slot frame can assume an on or off value. The coding is carried out in a time slot frame such that at least one time slot is preloaded or preoccupied with an off-value at a given position. The time slots that have not been preloaded have, at most, one time slot with an on-value in order to form a logic value of a double bit.
According to another formulation of the present invention, this and other objects are achieved by a mobile data memory and by a reader/writer for carrying out the method. Finally, in accordance with yet another formulation of the present invention, this and other objects are achieved by an identification system that has the reader/writer and at least one mobile data memory.
This has the advantage that the maximum data rate can be increased for the same operating distance between the reader/writer and the mobile data memory. Furthermore, for a predetermined data rate, the transmission distance between the reader/writer and the mobile data memory can be increased.